In a bowl, combine the soy sauce, oyster sauce, cornstarch, and water. Mix well and set aside.
Heat 1 tablespoon of vegetable oil in a large skillet or wok over medium-high heat. Add the sliced flank steak and season with salt and pepper. Stir-fry for about 3-4 minutes until browned and cooked through. Remove the steak from the skillet and set aside.
In the same skillet, add the remaining tablespoon of vegetable oil. Add the garlic, ginger, and sliced bell peppers. Stir-fry for 2-3 minutes until the vegetables are tender-crisp.
Add the shrimp and snap peas to the skillet. Cook for an additional 2-3 minutes until the shrimp are pink and opaque.
Return the cooked steak to the skillet. Pour the sauce mixture over the steak and shrimp, stirring to coat everything evenly. Cook for another 1-2 minutes until the sauce thickens slightly.
Drizzle with sesame oil before serving. Serve hot over cooked rice or noodles.